如何使以下代码更有效(例如,如何用查询替换循环)?
def get_question(datetime_now, questions_queryset, user):
best_schedule = None
best_question = None
# HOW TO ELIMINATE THE FOLLOWING LOOP AND REPLACE WITH A QUERY?
for question in questions_queryset:
try:
schedule = (Schedule.objects
.filter(question=question, user=user)
.latest(field_name='datetime_added')
except ObjectDoesNotExist:
schedule = None
if (schedule and (schedule.date_show_next >= datetime_now) and
((not best_schedule) or
(schedule.datetime_added >= best_schedule.datetime_added))):
best_schedule = schedule
best_question = question
return best_question
models.py
from django.contrib.auth.models import User
from django.db.models import DateTimeField, ForeignKey, Model, TextField
class Question(Model):
question = TextField()
class Schedule(Model):
datetime_added = DateTimeField(auto_now_add=True)
datetime_show_next = DateTimeField(null=True, default=None)
question = ForeignKey(Question)
user = ForeignKey(User, null=True)
最佳答案
您可以像在此答案 Subquery
中那样使用https://stackoverflow.com/a/43883397/3627387或与 Prefetch
一起使用https://stackoverflow.com/a/31237026/3627387
这是使用Prefetch
实现此目的的一种方法:
schedules_prefetch = Prefetch(
'schedule_set',
queryset=Schedule.objects.filter(user=user))
for question in questions_queryset.prefetch_related(schedules_prefetch):
try:
# using max here so it wouldn't do another DB hit
schedule = max(question.schedule_set.all(),
key=lambda x: x.datetime_added)
except ValueError:
schedule = None
这是一个使用
Subquery
的示例(它可能实际上不起作用,但是会给您大致的想法):from django.db.models import OuterRef, Subquery
schedules = (Schedule.objects
.filter(user=user, question=OuterRef('pk'))
.order_by('datetime_added'))
questions_queryset = (questions_queryset
.annotate(latest_schedule=Subquery(schedules[:1])))
for question in questions_queryset:
schedule = question.latest_schedule